For several decades, the electronics industry has made enormous efforts to shrink the size of electronic components and circuits with the aim of increasing the speed of operation and of reducing the power consumption/dissipation. These efforts have led to the development of integrated circuits and multi-layer ceramic devices which, in a volume of a few cubic millimeters, contain many thousands of transistors and other circuit components. These devices have very high operating speeds owing to the shortened distances the electrons need to travel inside of them. All of the modern circuits use advanced semiconductor materials, such as silicon and gallium arsenide, for example.
The discovery by Bednorz and Mueller (Z. Phys., B64 (1986) p. 189) of a new class of superconductor materials has of course opened another avenue to even lower power consumption and caused a worldwide search for possible applications of these materials in electronic circuitry. A number of studies on the electric field-effect in copper oxide compounds have been reported (for example by U. Kabasawa et al. in Japanese Journ. of Appl. Phys. 29 L86, 1990), but so far only minor field-effects in high-T.sub.c superconductors have been found. However, EP-A-0 324 044 already describes a three-terminal field-effect device with a superconducting channel in which electric fields are used to control the transport properties of channel layers consisting of high-T.sub.c superconductor materials. While this seemed to be a promising approach, growth studies of such devices have shown that in the suggested configuration the ultrathin superconducting layers readily degrade during deposition of insulator layer and top electrode.
In accordance with the present invention, this drawback is avoided through deposition of the superconducting film after the insulating layer, and locating the gate electrode underneath the insulator and the high-T.sub.c film. Still in accordance with the present invention, a conducting substrate is used as the gate electrode, and to facilitate the growth of preferably perfect crystals, substrate and insulator are chosen from the same crystallographic family of materials, that is, the lattice constants of the materials chosen to at least approximately match. For example, electrically conducting Nb-doped SrTiO.sub.3 is used for the substrate, and undoped SrTiO.sub.3 is used for the insulator layer.
The use of niobium-doped strontium titanate Nb: SrTiO.sub.3 in a high-T.sub.c superconductor structure was described by H. Hasegawa et al. in their paper "Contact between High-T.sub.c Superconductor and Semiconducting Niobium-Doped SrTiO.sub.3 ", Japanese Journ. of Appl. Phys., Vol. 28, No. 12, December 1988, pp. L 2210-L 2212, and in their EP-A-0 371 462. These references describe a diode structure where a superconducting film is deposited onto an Nb-doped SrTiO.sub.3 substrate. The authors of these references are only interested in measuring the rectifying properties and the resistance in forward and reverse directions. They "demonstrated that there are unknown interfacial layers between the two materials", a problem that the present invention elegantly overcomes.
This invention is based on experimental evidence for a significant electric field-effect recently discovered to exist in thin superconducting films. These experiments were performed with materials of the copper oxide class of superconductors, in particular YBa.sub.2 Cu.sub.3 O.sub.7-.delta.. Thin films of superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. are already known from EP-A-0 293 836. Epitaxial growth of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. is described in EP-A-0 329 103. For the purposes of the present invention, the value of ".delta." shall be considered to be close to zero (preferred), but it can be as large as 0.5. Those skilled in the art of high-T.sub.c superconductor materials will appreciate that many other materials in that class will be equally suited for the field-effect transistor structures of the MISFET type, as herein suggested. Also, other methods for depositing films of high-T.sub.c materials and of SrTiO.sub.3 are known in the art, such as laser evaporation, electron beam evaporation and molecular beam epitaxy.
While the acronym "MISFET" is usually employed to characterize Metal-Insulator-Semiconductor Field-Effect Transistor structures, this term will in the following description be maintained for describing an analogous structure, although the embodiments of the present invention to be described will use different materials, viz. electrically conducting Nb-doped SrTiO.sub.3 in place of the Metal, and a superconductor instead of the Semiconductor.
MISFET-type structures have been developed in accordance with the present invention which allow the application of electric fields larger than 10.sup.7 V/cm across insulating SrTiO.sub.3 barriers on ultrathin epitaxially grown YBa.sub.2 Cu.sub.3 O.sub.7-.delta. channel layers. Epitaxial growth of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. rf-magnetron sputtering is described in EP-A-0 343 649. In these structures, the normal-state resistivity and the density of free carriers in the YBa.sub.2 Cu.sub.3 O.sub.7-.delta. films can be modified substantially with gate voltages of on the order of 50 V.
Shortly after the discovery of the high-T.sub.c superconductor materials, Bednorz et al. in their above-cited EP-A-0 324 044 predicted on theoretical grounds that high-T.sub.c superconductor materials may bear an electric field-effect which is much larger than that in low-T.sub.c superconductor materials: The length scale by which electrostatic fields are screened in conducting materials is given by the sum L.sub.D +L.sub.DZ of the Debye length L.sub.D =(.epsilon..sub.o .epsilon..sub.r kT/q.sup.2 n).sup.1/2 and the width of eventual depletion zones L.sub.DZ =N/n. Here, .gamma..sub.o and .epsilon..sub.r are the dielectric constants of the vacuum and of the conducting material, respectively, k is the Boltzmann constant, T is the absolute temperature, q is the elementary charge, n is the density of mobile carriers, and N the induced areal carrier density. Because of their high carrier density, low-T.sub.c superconductors usually screen electric fields so well that the fields only have a minor influence on materials properties. To attenuate the screening, recent experiments on the electric field-effect in low-T.sub.c superconductors have focused on compounds with exceptionally low carrier density, like doped SrTiO.sub.3, with niobium as the dopant, for example.
In high-T.sub.c superconductor compounds, larger field-effects are expected owing to their intrinsically low carrier concentration and because of their small coherence length. The low carrier concentration of about 3 . . . 5.times.10.sup.21 /cm.sup.3 leads to screening lengths in the range of tenths of nanometers, and the small coherence lengths allow the fabrication of ultrathin layers with respectable critical temperatures. Superconducting films as thin as 1 . . . 2 nm have already been grown; electric fields can penetrate such films to a considerable extent.